P-hydroxybenzaldehydes are bifunctional molecules which participate a great variety of reactions characteristic of phenol and aldehyde groups. These aromatic aldehydes, particularly p-hydroxybenzaldehyde, are intermediates in the synthesis of various products by way of reactions of catalytic hydrogenation, oxidation, the Canizzaro reaction, reactions with compounds having an active methylene group, heterocyclic syntheses, reactions with amines and amides, formation of cyanophenols or ethers, halogenation, nitration, formation of phenylglycine or salts, and the like.
The most current uses for p-hydroxybenzaldehyde are in agrochemistry, the food industry, pharmacy, polymer and synthetic fiber industries and electroplating.
Industrially, according to the so-called Reimer-Teimann process, described in U.S. Pat. No. 3,365,500, p-hydroxybenzaldehyde is obtained from phenol reacted with chloroform and sodium hydroxide. This process leads to a mixture of ortho/para isomers, with a yield of 60 to 80% relative to the phenol. Although the two products can become independently valuable after separation by distillation, the cost is high. Further, the formation of sodium salts mixed with unreacted phenol requires a recovery treatment of the initial product that is economically unprofitable, and a treatment to make the effluent ecologically acceptable.
According to patent U.S Pat. No. 4,119,671, condensation has been proposed of formaldehyde on formol in a basic medium which leads mainly to a mixture of 2- and 4-hydroxymethylphenols, which are then catalytically oxidized with oxygen into aldehydes. However, this process proves difficult to use because of the great reactivity of the formaldehyde/phenol system which causes the formation of very complex mixtures. Further, the use of this process is limited by the difficulty of separation of two hydroxymethylphenols from the mixture in the presence a base. Yet this operation, which reduces the overall yield of the process, is essential prior to the oxidation stage.
Oxidation of p-hydroxytoluenes to p-hydroxyaryaldehydes by molecular oxygen has also been considered. These oxidation reactions are catalyzed by organic and inorganic salts of cobalt, manganese, chromium or nickel in the presence of sodium hydroxide in methanol. The yields vary from 40 to 80% and depend on both the substrate used and the operating conditions. Following this type of process, European patent application No. 0012939 advocates obtaining derivatives of 4-hydroxybenzaldehyde in the presence of a catalytic amount of a compound of cobalt or metallic cobalt, indicating rather high rates of conversion or selectivity but in an overall manner without isolation of the desired product.
To make obtaining of p-hydroxybenzaldehydes economically interesting and ecologically acceptable a process must be sought that uses a cheap starting product, comprises a minimum number of reaction stages, within the bounds of possibility without intermediate separations, operates under mild conditions, leads to a single isomer with a high conversion, and releases effluents which contain only small amounts of organic material and mineral salts.